The interband cascade laser is a promising semiconductor coherent source for the midwave infrared (mid-IR, defined here as 2.5-7 μm) spectral region, with extension to longer wavelengths a possibility if the internal loss that increases with increasing wavelength can be suppressed.
The ICL differs from the mid-IR quantum cascade laser (QCL) by employing interband rather than intersubband active transitions. The carrier lifetime associated with interband transitions is three orders of magnitude longer than for intersubband transitions, which ultimately results in more than an order-of-magnitude lower drive power in the ICL.
The basic concept of the ICL was proposed by Rui Yang in 1994, while he was at the University of Toronto, and patented shortly later while he was at the University of Houston. See U.S. Pat. No. 5,588,085, “Light Emitting Devices Based On Interband Transitions In Type-II Quantum Well Heterostructures” (1998). Subsequently, critical improvements to the basic ICL structure, such as including more than one hole well to form a hole injector, were made in a joint patent by two of the present NRL inventors and Dr. Yang. See U.S. Pat. No. 5,799,026, “Interband Quantum Well Cascade Laser, With A Blocking Quantum Well For Improved Quantum Efficiency” (1998).
This was followed by a number of other patents by some of the present NRL inventors, which introduced numerous further improvements. See, e.g., U.S. Pat. No. 5,793,787, “Type II Quantum Well Laser with Enhanced Optical Matrix” (1998); U.S. Pat. No. 5,799,026, “Interband Quantum Well Cascade Laser with a Blocking Quantum Well for Improved Quantum Efficiency” (1998); U.S. Pat. No. 8,125,706, “High Temperature Interband Cascade Lasers” (2012); U.S. Pat. No. 8,290,011, “Interband Cascade Lasers” (2012); U.S. Pat. No. 8,385,378, “Interband Cascade Lasers” (2013); and U.S. Pat. No. 8,493,654, “High Temperature Interband Cascade Lasers” (2013). One of the most critical of those improvements was to substantially increase the doping density in the electron injector, so as to increase the ratio of electron and hole densities in the active QWs and thereby lower the lasing threshold current density (“carrier rebalancing”). This dramatically reduced the threshold power for ICL operation, to as low as 29 mW at room temperature. See U.S. Pat. No. 8,798,111, “Interband Cascade Lasers with Engineered Carrier Densities” (2014); and U.S. Pat. No. 9,059,570, “Interband Cascade Lasers with Engineered Carrier Densities” (2015).
ICLs have multiple active gain stages that are separated by electron and hole injectors. Most commonly, the active optical transitions that produce gain are spatially indirect (type-II), in which the electron and hole wavefunctions peak in adjacent electron (e.g., InAs) and hole (e.g., GalnSb) QWs, although they may also be spatially direct (type-I), in which the electrons and hole wavefunctions peak in the same layer (e.g., InGaAsSb). See U.S. Pat. No. 5,799,026 and U.S. Pat. No. 9,059,570, supra.
Most of the laser ridge waveguide and contacting architectures applied so far to ICLs have followed general principles known to the semiconductor laser community. To provide optical confinement, most ICLs employ moderately n-doped InAs/AlSb superlattice cladding layers on top and bottom of the active gain stages. Most designs also insert top and bottom separate confinement layers (SCLs), consisting of lightly n-doped GaSb, between the active gain region and the top and bottom cladding layers. The typical ICL layering design, which usually includes a bottom GaSb substrate and buffer layer, bottom cladding layer, bottom SCL, three to fifteen active gain stages, top SCL, top optical cladding layer, and heavily-doped cap for electrical contacting, plus thin transition superlattices inserted between the various regions to assist electrical transport from one region to the next, is fully described, for example, in I. Vurgaftman, R. Weih, M. Kamp, J. R. Meyer, C. L. Canedy, C. S. Kim, M. Kim, W. W. Bewley, C. D. Merritt, J. Abell, and S. Höling, “Interband Cascade Lasers,” J. Phys. D 48, 123001 (2015).
The primary role of the n−-GaSb top and bottom SCLs is to place a substantial fraction of the lasing mode in a region with lower loss than either the active gain stages or the optical cladding layers. Nonetheless, free carrier absorption in the cladding layers contribute to the net internal loss, since those layers must be doped heavily enough to provide adequate electrical transport for current injection. Furthermore, as discussed below, the loss can increase substantially when the lasing mode must strongly couple to a distributed feedback (DFB) grating that is etched into the top of the ridge waveguide or into a dielectric layer that is deposited on top of the ridge waveguide.
Distributed feedback (DFB) interband cascade lasers that emit in a single spectral mode are attractive mid-IR sources for use in chemical sensing and other applications. DFB ICLs spanning the wavelength range between 3 and 6 μm have recently become available as commercial products. See http://nanoplus.com/en/products/distributed-feedback-lasers/distributed-feedback-lasers-3000-nm-6000-nm/. However, none of the DFB ICLs reported to date have displayed power conversion efficiencies approaching those of non-DFB ICLs. See, e.g., M. Kim, W. W. Bewley, C. L. Canedy, C. S. Kim, C. D. Merritt, J. Abell, I. Vurgaftman, and J. R. Meyer, “High-Power Continuous-Wave Interband Cascade Lasers with 10 Active Stages,” Opt. Expr. 23, 9664 (2015).
The most straightforward geometry for realizing a semiconductor distributed-feedback (DFB) laser emitting in a narrow spectral line is to etch a grating into the top surface of a structure with reduced top optical cladding thickness that allows penetration of the optical mode into the grating region. See, e.g., C. S. Kim, M. Kim, J. Abell, W. W. Bewley, C. D. Merritt, C. L. Canedy, I. Vurgaftman, and J. R. Meyer, “Mid-infrared distributed-feedback interband cascade lasers with continuous-wave single-mode emission to 80° C.,” Appl. Phys. Lett. 101, 061104 (2012).
Some semiconductor material systems used for DFB lasers then allow the overgrowth of an electrically-conducting semiconductor with lower refractive index than the active core to complete the top optical cladding layer and also provide a low-loss pathway for injecting electrical current into the semiconductor. However, no suitable overgrowth capability is currently available for interband cascade lasers (ICLs) grown on GaSb or InAs substrates. Therefore, for ICLs the top of the grating itself must be metallized to provide the top electrical contact. The coupling coefficient varies exponentially with the top cladding thickness. For example, for an ICL emitting at λ≈3.5 μm, the top cladding thickness must be reduced to at most ≈500 nm if optical coupling to the grating is to be sufficient to suppress non-resonant longitudinal modes and assure the production of a narrow linewidth.
Thus, a primary disadvantage of this geometry is that without an overgrown spacer layer, the lasing mode penetrates into the metal contact, introducing additional optical loss that also varies exponentially with the cladding thickness. While the magnitude of the extra loss depends on the contact metallization scheme, a typical value for ICLs with conventional Ti/Pt/Au contacts is at least 2-3 cm−1. Since this is comparable to the internal losses originating from all other sources combined, the mode overlap with the lossy metal inevitably degrades the slope and wallplug efficiencies. Furthermore, this loss may increase substantially at wavelengths longer than 4 μm.
While alternative DFB geometries such as side gratings which sometimes employ a double mesa are possible, those approaches have other disadvantages related to coupling strength to the grating, processing/performance yield, and/or maximum single-mode output power and efficiency. See Igor Vurgaftman, William W. Bewley, Chadwick L. Canedy, Chul Soo Kim, Mijin Kim, Charles D. Merritt, Joshua Abell, and Jerry R. Meyer, “Interband Cascade Lasers With Low Threshold Powers and High Output Powers,” IEEE J. Sel. Topics Quant. Electron. 19, 1200120 (2013); and Michael von Edlinger, Julian Scheuermann, Robert Weih, Christian Zimmermann, Lars Nähle, Marc Fischer, Johannes Koeth, Sven Höfling, and Martin Kamp, “Monomode Interband Cascade Lasers at 5.2 μm for Nitric Oxide Sensing,” IEEE Phot. Tech. Lett. 26, 480 (2014) (side gratings); see also S. Forouhar, C. Borgentun, C. Frez, R. M. Briggs, M. Bagheri, C. L. Canedy, C. S. Kim, M. Kim, W. W. Bewley, C. D. Merritt, J. Abell, Vurgaftman, and J. R. Meyer, “Reliable mid-infrared laterally-coupled distributed-feedback interband cascade lasers,” Appl. Phys. Lett. 105, 051110 (2014) (double mesa).
However, a significant issue in general for DFB ICLs is that the grating coupling strength tends to be marginal at best, because the evanescent coupling to a side grating is inherently weaker, and because the stronger coupling possible with a top grating is inevitably accompanied by a higher optical loss associated with mode penetration into the contact metal deposited directly on top of the grating. Marginal coupling to the DFB grating leads to degradation of the yield and tuning range for lasers emitting in a single spectral mode. The issue is amplified further when a higher-order grating is desired to relax the requirement for high spatial resolution in the processing, for example, to allow optical rather than e-beam lithography to be employed.
For non-DFB designs, it is usually preferred that the optical cladding layers be thick enough to fully confine the lasing mode so as to minimize the optical losses associated with evanescent penetration of the mode into the top contact metallization as well as losses resulting from leakage of the mode into the GaSb substrate whose refractive index is higher than that of the active gain stages.
However, when a DFB grating is present, the top cladding layer must be thin enough to provide substantial overlap between the lasing mode and the grating. As noted above, in conventional DFB ICL designs that call for deposition of the top contact metal directly on top of the grating, this inevitably increases the internal loss because the lasing mode evanescently penetrates into the lossy metal contact.
A recent patent application filed by some of the present inventors disclosed a method for mitigating the high loss associated with penetration of the lasing mode into the contact metal, namely by depositing the metal contact only on some fraction of the top of the ridge (reduced contact fill factor) rather than the entire top surface, and then relying on the strong current spreading characteristic of ICLs to provide current injection into the lateral regions not covered by the metal. See U.S. Patent Application Publication No. 2016/0359298, “Interband Cascade Lasers with Low-Fill-Factor Top Contact for Reduced Loss.” Preliminary reductions to practice confirmed that the method effectively lowered the lasing thresholds and increased the slope efficiencies of ICL test devices.
However, that approach cannot reduce the loss to zero because some fraction of the mode still overlaps the contact metal. There is also a limit to the minimum thickness of the top optical cladding layer, since even with a reduced contact fill factor the loss induced by mode overlap with the remaining contact areas becomes excessive if the top cladding layer is too thin. That places an upper bound on the maximum coupling coefficient between the lasing mode and the DFB grating. It would be preferable to eliminate the mode overlap with the top contact metal altogether, so that a much stronger grating could be provided without the usual penalty of high optical loss.
While ion bombardment has proven to be a useful tool for suppressing both optical gain and electrical conduction in diode lasers emitting at wavelengths, no previous works have described the effective application of this approach to interband cascade lasers. ICLs present special challenges differing from those of other semiconductor lasers, especially concerning the difficulty of suppressing the lateral injection of current into regions that are not intended to provide gain. Previous experiments at NRL confirmed that ion bombardment can fully suppress lasing and photoluminescence in interband cascade laser structures. See C. S. Kim, M. Kim, W. W. Bewley, J. R. Lindle, C. L. Canedy, J. A. Nolde, D. C. Larrabee, I. Vurgaftman, and J. R. Meyer, “Broad-Stripe, Single-Mode, Mid-IR Interband Cascade Laser with Photonic-Crystal Distributed-Feedback Grating,” Appl. Phys. Lett. 92, 071110 (2008). However, NRL attempts to significantly reduce the lateral electrical conduction in ICLs were unsuccessful. We are not aware of any prior experiments investigating the effects of ion bombardment on interband cascade laser properties.
Another issue that often affects the ICL processing yield is current leakage at the sidewalls of a narrow ridge, resulting from damage imposed by the reactive ion etching process that defines the ridge. NRL has employed both CH4/C12- and BCl3-based inductively coupled plasma reactive ion etch processes. See M. Kim, C. S. Kim, W. W. Bewley, C. D. Merritt, C. L. Canedy, J. Abell, I. Vurgaftman, and J. R. Meyer, “Interband Cascade Lasers with High CW Power and Brightness,” Proc. SPIE 9370, 9370029 (2015). While the CH4/Cl2-based etch has generally been somewhat more reliable, even with that process the yield associated with reactive ion etching remains a significant issue. The invention will substantially mitigate this yield issue.
A further consideration well known to the art is that for a fixed ridge width, a narrow ridge waveguide semiconductor laser tends to operate more reliably and stably in the fundamental lateral mode if it is “weakly index-guided”, that is, the mode is confined to an inner ridge that is defined by a relatively weak lateral spatial variation of the refractive index rather than by gain guiding or by a more abrupt lateral variation. An abrupt lateral variation of the refractive index is obtained, for example, when the ridge width is defined by a single deep etch to below the active quantum wells. The ridge sidewalls are then typically coated with a dielectric that has much lower refractive index than the semiconductor epilayer. It is known that a more preferable variation of the modal refractive index is obtained when a shallow etch is employed to form an inner ridge to laterally confine the lasing mode. Weak index guiding is readily applicable to most types of semiconductor lasers, since lateral current spreading is minimal and current injected into the inner ridge remains confined mostly to the inner ridge.
However, it is known (see, e.g., Forouhar et al., supra) that lateral current spreading tends to be quite extensive in an ICL, because of the very large electrical anisotropy of the short-period InAs—AlSb superlattice that forms the upper cladding layer. While a double ridge geometry was applied to DFB ICLs by Forouhar et al., the threshold current was excessive compared to most ICLs due to lateral spreading into the outer ridge that does not contribute to gain. For this reason, nearly all other previous narrow-ridge ICLs have employed a single ridge defined by a deep etch to below the active gain stages. As mentioned above, this induces strong index guiding that degrades the mode properties associated with a ridge of given width.
The present invention provides a means for realizing weakly index-guided ICL ridges in which current does not parasitically spread appreciably into the outer ridge.